Light spectrum dispersion effect in Blender?
It is fake-able to some extent in cycles, see some of these results:
http://www.blendernation.com/2012/04/05/cycles-diamond-dispersion/
http://www.blendernation.com/2012/09/04/cycles-dispersion-glass-shader/
One technique I know of is to use multiple Glass shaders with different IOR values
A quick setup:

Low sample result:

(Not that great, but the closest method I know of..)
Luxrender is an open source unbiased bi-directional rendering engine with blender integration (and GPU rendering support via OpenCL) and is capable of this.
Some examples:
http://mitsuma.deviantart.com/art/LuxRender-Prism-198927185
https://www.youtube.com/watch?feature=player_detailpage&amp;v=kIX5WL07Uss#t=15s
Luxrender (technically SmallLuxGPU, an early version of luxrender with GPU support)
https://www.youtube.com/watch?feature=player_detailpage&amp;v=jk-N4f9ze4k#t=82s
Yafaray is another open source render engine with blender integration that supports both biased and unbiased techniques, and also seems to handle this well.
http://www.youtube.com/watch?v=RVHYMq8NEEs